19s) 215-233 :. -- : - -.~ , . . .- -,;: .-.. .- . . :- .-, .,: -, ‘25; :- 1...

chemical Pl&ics 94 \19S) 215-233 :. -- : -_ -.~ ,_ . . .- _ -. .-- - _ -. .. : : _ :_. __..:-.- i -: -,;:_ _.- . . ._:- .-, .,: -, __ .-.. ‘25;

North-HoUand.Atmterdam :- 1 _’ ._ _-, - ‘I. _ .::-- -.I -_ -.. :. : . [ .A.r,_-,. ;, --. :-._ ..T : -.- _( .~ ;._i _ .y*-i; ._ :

. . -: . -_-..I- . . ~._.. -. .- ~. _> : ; __ _. ; . _ ._ : -_: 11 ,y: .- : ~: _:: ,. ._ .~ .’ _. ,-.: -. ‘.

.- _ : ._ ._ -..- _ -. _~O&G&;~~~. INTER&C&ION !TkJDY OF SHAEE~SIRU.

. -_.n- - CfuRE :.~ -_ -. .._ _; _.- :-. ;.--.;.

: ACCOMP ANYiNG CORE-IONIWTION CBFSW -ARtiMAT& MOLECULES:;:’ :-- -rNTHEvAPGBANDCoNDENsEDPrGSES: .m - .-. .-

_, .~.-_ -,;

~OSOBENZENE;DIAZOB~~.DIOXLDEAND’~-NO.DIMER’ - -.-.-__.. -I- ;i,

-H--J&&~~D ._~. -. .-. ..:- -- 1 : .- : : : _. :.

Insrimt fSs PhysikaIisdre md Ilharetishe Chemie der (InicmGit Eriangm-.&ii -. -- _ E~e3.852OErlangm.FRC

.

Richard W. &GEL& -. :

.%-ox Webxer R&c& Cenrm. &oo Phillip Road 0114-390. W&t&. Ah- York 14580. (iSA -- :

and

Birgitt BORSCH-PULM and Harald PULM Instint! Jiiii Organische Chemii der lh&-sit~ :u Ki%. Gmhsrmrrr 4.5ooO Cologne HI. FRG

Rcodvcd 16 July 1984: in final form I2 November 1984 - .~ :

Manifestations of the condensed phase dimcr formation on the N Is and 0 Is corehole shake-up stxwxure of nitmsoben- zaw arc examin& thcorcticaUy within +c CNDO/S CI cquivaknt~re app roximation Computations on the nitrosobcmene dimcr (diarobemene dioxide) were paformai to simuktc the solid state cnviro- t Results obtained using singly excited and singly and doubly excited (“sin&t--roupled) ~k&fig~tions arc contrasted to &nphasizc the importance of cxr&tion contriiutioux Although i&&& interactions are found to .suo&ly de&nine the F~UIE of the solid s&ate sp&tra. rhe final state cnugy and intensity distriiutions arc similar for the vapor (monomer) and andused phase (dim@ system&as obscnwi uperimentalty- This “accidental” corxspondcncc is discus& in terms of component conuiiutions to the qxctxal functions Computations on the N Is and 0 Is h@e states of the nitric oxide (NO) dimer yield results in close agmment u=i* cxist;ng amdcnscd phase spectra of NO_.Vti~~similtities between the corehole induck -inkrmokc&~ issxt.iOns in the &uosobaatnt dimer and nitric oxid: dimcr sysans arc noted and disks&F&y. compatison of the UPS spcc@m of condensed phase nitric oxide witb~appropriate vaknoz level kakulations ok’ the dimcr also indicates strons hol&nduccd “int&mokcdar” effects amsistmt with the yyly-sis of the oxc-level region 7

:

: Lrntroduction -0ile more G+I expected in a system with only two.

typei of nitrogen A single peak was attributed to Recently it itas be& recognized that core-ioni- the amino group, whereas two components with

ration spectra of- mokcules in the gas phase may : e+alIy &igb intensity were found for ionization of differ considerably from those obtained on the the -N$ group_- There is generalagreememwith condensed molecular solid [l-4]_ Not only does the original assignment of.the PNA qn&rum given the binding energy of the core-hole state change by Pignataro and i Distefano. a [S] that: the :latter upon .-condensation,. but the Complete spectral feature arises from a shake&p excitation i.nVoIGng function may be altered. p-n@roanihne (PNA) is c&&&t trrkfer from 1 de. :.a&iqo (donor) to : he _. the most- prominent exampIe of such-a system In nit.15 (acceptor) group. The intensity and ene$y of the-case of.condensed~PNA three kistimxpeaks this shake-up :. excit&on &a&e& .; considerably, _ s observed’& the N 1s jonizatioti spktrum/ 0; howewz.r,_ if the. spectrum is taken, OnI- &e.I:+&por _;_- .-. .-

~301-01&/85~$03.& Q Blsevier Science Publishers B-V_ (North-Holland Physics Publishing Dhision)~ ;- : ..___ : : -’ -- _ -. :i .I .~_ .:. ‘:: _::

.

phase species as shown by Banna !I]_ Bigelow and Freund (2.31 proposed au explanation of this effect in terms of intermokcular screening induced by strong coupling of the !ocaIized ion state to the surrounding neutral molecules: It turned out that calculations on dimer systems resulted in spectmI changes relative to the free moIecuIe which were very similar to those observed experimentahy when

going from the vapor to the solid phase, provided that correIation contributions were included in the analysis [6]_ It was possible to show that the active coupling mechanism was not hydrogen-bonding as had been suegested [6]_

So far PN_4 and 2-amine-6-nitronaphthalene [6] are the only eztmples where both vapor and condensed phase spectra. as well as a series of appropriate caIct~Iations. have been reported_ To further elucidate the electronic properties leading to vapor-tocondensed phase spectraI changes it is worth carrying through an equivalent study on an additional system

Vety recently Slaughter et aI_ reported the phase core-ionization of nitrosoben- which provides with the

ing solid spectra of et al_ another set spectra on to test solid state interactions reasonably the observed in the function Nitrosobenxene particularly weII for such

study because is known dimers are ally formed condensation [9.10]_ monomeric were detected a X-ray of the [ 11 ]_ The dimer is formed by coupling two nitroso groups via a strong N-N bond with 1_32 A bondlength [ll]_ The crystaI exclusively contains dimer molecuIes with a c5s-configuration- This dimer formation is accompanied by a rather strong distortion of the local nitrosobenzene geometry compared to the planar structure of the monomer in the gas phase [12]_ The trans-configuration, where the NO-b enzene moiety should have a structure simiiar to the gas phase. is not found in the crystal although it is not certain whether this configuration is formed in a frozen vapor (ll]_ However. there is no doubt from an experimental point of view that specific and strong dimer interactions exist in the crystalline solid

It is therefore somewhat surprising that the core

photoelectron spectra of gaseous and solid nitrosobenzene are rather similar_ The purpose of this work is to address this apparent ambiguity_ In addition, unlike the case of PNA, where “pairings occurs between adjacent donor/acceptor moieties (not dimerization in a chemical sense). nitrosobenzene exhibits “strong* dim& interaction between atoms on adjacent acceptor groups- The question is: does the N-N bond formed on condensation influence the cot-c-hoIe spectra; and if so. why are gas and solid phase spectra so similar7 In this connection it is useful as we shall see in the course of this paper, to compare the spectra of nitrosobenxene dimers (d,,= 132A (111) with those of solid NO [13,14], where dimers are formed in the solid with a N-N bond length of = 2.2 A [15,16]_ Recently, Tanner et aI_ 1131 reported strong changes in the spectral function upon condensation of NO which were interpreted as being due :o intermolecuku screening within the NO dimsr moiety_

We report in this paper the results of calculations on monomeric and dimerized nitrosobenzene (&-and trans-configuration). as well as a brief excursion on the relationship to NO dimers, to show the intluence of intermolecular bond strengths on the spectraI function. Our computations indicate that the simiIarities between gas and solid phase spectra are “accidentaI”_ The nature of the transitions involved in the satellite excitation are significantly different_ While in nitrosobenzene the charge transfer between sub- stituent and phenyl group is the important screening prw the intermoIecuIar charge transfer between NO moieties is the dominating .mechanism infhxencing the satellite intensity within the nitrosob enxene dimer- In comparison to the nitric oxide dimer the nature of this intermolecular screening process is d%cussed_ It is shown that electron correlation is im_portant in correctly de- scribing satellite intensities_

L Computational

Calculations were performed withitrthe semi- empirical all-vaIence-eIectron closed shell CNDO/ S formahsm [17,18] including up to doubly excited

configurations [19) using the Pariser-Parr approximation (201 to the two-center CouIcnib rep& sion integrals_ Core-ionized species were treated within the equivaknt-core approximation [2&q where the atom to be ionized (atomic number 2) -was replaced by the next highest atom in the periodic table (Z’ = Z + 1). Convergence was achieved for aI N 1s and 0 1s monomer and dimer geometric configurations considered. Excited state computations were then performed on the “equivalent-core” closed shell species assuming only singlet coupling between alI valence electrons. In all core-ion state cakulations which include doubly excited state configurations the 300 lowest energg excitations were selected from 5000 created configurations of proper symmetry Relative intensities, IL, were obtained within the sudden approximation by projection of the correlated ion- state wavefunction (!I$&~) onto that of the “frozen’ ion state (*lF_) given in terms of the one-ekctron orbitah of the neutraI system,

Projections of the relaxed ion-state wavefunction onto the correIated and uncorrelated initiaI state were performed to examine the importance of correlation contributions [6,23]_

The calculations on the valence-ionization spectra were performed using an open shell version of the CNDO/S program 1241. We calculated the iowest energy doublet state of the radical cation self-consistently and then treated the higher excited doublet states by configuration interaction in the basis of determinants created from the one-electron excitations of the ground doublet state_ The intensities for the ionization spectrum were caIcuIated from the projection of ground and excited doublet states onto the determinant of the neutral species [25]. For the valence spectra only sir&y excited states are inchtded-

Molecular coordinates used in the computations were calculated from bondlengths and bond angles given by Pople and Beveridge [26]_

Using our semi-empL+aI wavefunctions we calculated electron de&&y diagrams and electron density difference diagrams [23]_ We ,aIso cakulated bond indi& according to a prescription

given by Cohen [27] as dkussed in de&I elsewhere

WI-

3_ Results and dkussion

3_ I_ Nirrosobenzene monomer (N3)

The computationaf results for N 1s and 0 Is ionization of the nitrosobenzene monomer (NB) are compared in fig 1 to the e.xperimentaI gas phase spectra given by Slaughter et aI. [7]. Since neutraI mokcule correlation contributions had Iit- tle effect on the find state spectra, only the results obtained by projection of the ion states onto the uncorrelated neutral ground state are shown. The computations clearly reflect experiment, particularly with respect to the differences between N 1s and 0 1s ionization_ For example, a satellite is obtained at 2.6 eV for N 1s ionization and 3.6 eV for 0 1s ionization_ These energies are in close agreement with the experimental values of 243 eV and 3-75 eV, respectively 171. The experimental 0 IS shake-up features appear asymmetric and broader than the main line, whereas the shake-up band associated with N Is ionization is consider-

ably sharper Such reIative behavior is reproduced in the computations_ As indicated in fig_ 1 the asymmetry in the 0 1s shake-up spectrum arises from a second relatively intense excitation not present in the N 1s rest&s_ In addition, the broad shake-up feature observed in the 0 Is spectrum between 10.0 and 15.0 eV is’reflected in the computational rest&s. Although a similar intensity pattern is predicted for N 1s ionization, the experimental spectrum was not extended to sufficient energies to aIIow a similar correlation

Locahzation of the orbitak which contribute most strongly to the shake-up interpretation is given in fig. 2. Within the present set of one-electron orbitak the highIy excited configuration (19 --, 21) separately accounts for only a smaII portion (8%) of the shake-up intensity as indicated in tables 14_ The shakeup transitions thus *‘gain’ the appropriate additional intensity through con- structive interference between the highly excited configuration, the ionic ground state and the doubly excited configurations in the final state inten-

Nls 01s

Nls - HOLE NEUTRAL Ok-HOLE

i

923

- 5.037 eV w

llnou

em -1630 ev

Q 16 -2S6l eV

Q22 CP -Xi03 eV

Q20 lQ2 eV

Q 19 _fY -n276ev

_

I f I f

.I I i ck+f

921 4954 eV

it? of final state interaction is emphasized by the fact that the higher-lying transitions of- tables l-4

cannot be rep~ntcd ii terms, of a_ domixignt component_ . . . .,~ . .

jdistefano et iii [S] ~reported the results of

Table I. The encqks and wavefw&O~ of the ion states of--&

nib mono- labelled as indiciccd in fig l_ The

energies are r&&e 10 the uncorrelat+ co&-ionized ground

stat= The wavefunctions are characterized by their CI co+-

cians and the correspomiiq dcrerminams j+xn as a set of four numbax in bmekeu representing orbit& of the eqtiv-

aknt-eore ground stak Occupied and tmoceup ied orbitals are

separated by a double Gash

state Energy l&l

Wawzfunction

1 - I.1776 -0-90 904 (O.OgO.0)

-0.17 264 (0.19~1.0)

G-029 250 (19.191p12l) ~0.10 779 (16.19Lp121)

7 l-4807 029 483 (O.Olp.0) -0-79 810 (0.l9lCI.O)

-0.12 no (0.19Lg2.0)

+0_11275 (192OIrn23)

to-4 199(19.19&2121)

3 3_sws -0.E 139 (O.O~.O)

- 0.37 387 (O.lS@ZO)

t 023 924 (02Og3.0)

+0_4124!j (0.16~1.0)

-02 3% (19.19~I21)

io.4777 (19.19@&22)+ --- 4 6J300 -0.49 497 (OJ9lprO)

i 0.34 998 (02CIp3.0) -03 117 ([email protected]) -0_36406(19,19~121)i- ---

Table 2 Owriap integrals between the ionic and neural mokctde SCF detaminants for N 1; ionization of the nilrosobenzenc mono- -

c0.0Qo.0) (oJ9rw.O~ (1939K2ul)

vdslate

O-7861 --0*1-

O-06765

CNDO/S CI caIculafio+ on the nitrosobenzene monomer where-only singly excited configurations were considered- The one-particIe.ene-rgics c&u- lated by Distefano et al_ -differ from.-ours due to variations in $ram&xiz+iion_ The N 1s and 0 1s

shake-tip energies obtained .&hitt the singly excited scheme are higher-than *e present values by = 1.0 and 0.7 eV; respectively_ Iti addition, the N 1s and 0 1s shake-tip iittensities given by Diskfano et al_ are considerably smaller than those shouin in fig- l_:Wetattribute these efekences to the-inclu-

__^ m--m a__

~l~.qw&~ _ __ y U.UI

sion of .do&ly excited con@urations in the pres-

(f&l6Ip1.?) ;_ ; -fin* “_“s49 - : ent analysis_ Nevertheless, the present i&rpr+a~ tidn as to the nature of the most intqse &i&e-tip

1 :..-0.82. -0.96 007 (0,Olp.O) : ‘~ :_:

-2 2_71a4 I‘

GO.17 504<19.19~121) : :-. :_ 0.12 173 (O.Ojp.0). ‘-

+cis5 473 ([email protected]) ,-.. i-o.17 982 (020=0)_’ :_

i-O.31 855 (lM9~21)

-0.17 185 @[email protected]) 3 4.7906 0-58%3(020lP.0) _- _

i-p.3 937 (OJ9103.0)

-0.36 319 ~;r5~~1.0) -_

-0.37 144(202O@l_2l) . . 4 5.5536 0.30693 (02O@XO)

+ 0.40 529 (O.lS~l.0)

i-o_55 818 (192O[j21.22)-

-033 3’17QO20@12l)-r ___ I

TabIe4

chrrfap iatcgralc betaren the ionic and neutral moleeuIc SCF determinants for 0 Is ionization of the nhosobeozene mono_

mef

Ion con@ration Ovqhp aifh neutral

ground slate

(O.Oip.0) O-70 983

(19J9gx21) 0.136 309

(0.19w.o) 0311056

(02OgzO) -O_OIZ 688

(0.191F3.0) 0.0168743

(ro$Or&=) o.oooo24

(192O[F122) O_oooMM (OJwLO) 0.143 802

features parallels the assignment given by Dis- tefano et aL [St_

To illustrate the natm-e of the ion states which lead to the N Is spectral function appropriate electron density difference plots are given in fig 3. Fig 3a shows the difference betweeu the core-hole ground state and the ground state of the neutral system_ SoIid lines represent electron gain in the ion, whereas dashed lines correspond to electron Ioss_ The surface on which the electron densities are in&at& Iis 05 A above the molecule in order to include changes in ~-electron density for which the moLzcuIar plane corresponds to a nodal plane. CIearIy the plot shows a screened ion ground state where a screening charge of 0.803 e ac- cumulates on the ionized nitrogen_ The gross atomic popuiations. as weli as the bond orders calculated according to Cohen% 12728) proposal are given in fig. 4_ Obviously the entire molecule contriiutes to the mg of the ionized center- Fig_ 3b indicates, however, that upon eIe4Xronic excitation 024 e is transferred back from the NO

moiety towards the benzene rins It should be noted that an interpretation of the shake-up process in terms of the orbital localizations given in fig-2 suggests just the opposite direction of charge transfer, i-e_ from the aromatic ring toward the NO moiety_ The origin of this counter-intuitive result lies in the fact that the final excited state is not pure, but rather a mixture of five determinants (tabIe 1). The charge distribution is given by the square of the complete wavefunction, including interference terms- In this case the cross terms dominate_ Decomposition of the excited state charge transfer into -atomic components reveals that the oxygen atoin contriiutes 0_2 e (75% of the total excited state charge transfer). whereas the nitrogen looses only O-04 e (25% of the c*e transfer)_ The ionized N atom then carries only 5% less screen&g charge in tbe exdted state than in the ionic ground state Shake-up excitation yields an UEcreened oxygen center, even with respect to the neutral ground state_ The manifestation of charge transfer from the oxygen atom on -shake&p

!+_4_Ammicckarondensities(drrkd)andbond orders (s;-bond order i& pamnthcscs) i?rLhe uncot-rektai gnbmd ~tarcsofthcmuualN ls ionizeda& 01s ionizcdnitroscF banaKnmrlomas. . . . _. ~-

excitation is quite similar to the situation. found for N 1s (NC,) .ionization of. PNA : It -shouId:b;e noted, however+ that in PNA- excited stat&-charge. transfer from the oxygen is tOthe center of-io&a- tion r&r th& the ring In PNA it is interfere& betweenthe.NOa (oxygeh) z&d NH, charge u-an& fer that induces the intense satellite_ This explains why n$robenzene, where onIy oxygen transfer is possible, does not exhibit intense -satellite ~st.rucf ture The electron density difference plots for the higher-lying shake-up satellites of~interest are given in figs_ 3c and 3d These ~&e ~characterized by a charge transfer from the benzene ring toward the_ NO moiety- In these states the nitrogen atom carries a screening charge Iarger than in the ionic ground state. The “extra’* screening in these states is deiivered by *he t electrons in the C-C and the C-N bonds. One can speculate that the-molecule dissociates when :hese states .are excited.

Figs. 5a-5d. show the corresponding electron density difference plots for .O 1s ionization. The core-ionized ground state (fig Sa) carries a-screening charge of 0.708 e on the oxygen center. The nature of the most intense 0 1s satellite par&& that found for N Is ionization The orbitak of the 0 1s core-ionized species again sug@st a rather massive charge transfer from the phenyl -ring toward the NO moiety (see fig 2 and table 3j_ Due to the nature of the cross terms appearing in the charge density evahration, interference effects Iike those noted for N 1s ionization inhibit excited state charge transfer_ AS indicated in fig_ 55, there is essentially no ner charge transfer_ The ekctron density difference given in fig. 5b can be viewed as a “charge density osciIIation” moving between the extremes of the molecule The higher excited 0 Is satellites also exhibit sim&rities with the N. 1s results Again, there is negIigibIe charo,e transfer between the pheny1 and NO groups. The. second shake-up state is more intense than the corresponding N -Is-trat&ion because the projection of orbital #15 onto the LUMO IeveI is much larger. The satehites aazompa&ing 0 Is-ionization ap- pek at higher energy than the corresponding N 1s features due to a larger HOMO-LUMO gap and the stronger stabihzation Of orbital ,‘19_ This htter selective interaction is caused by the. con: tracted oxygen functions tihich Ied to an enhanced

Fe 5_ Eleuon dmuity dirruaur maps of varioos groond and excited smtcs of the 0 Is ionirat niu~~bcnzeae monomer (see text).

Coulomb stabiJ.ization upon ionization for the levels kaIized on the NO moiety_ It is worth

noting that the differences in HOMO-LIMO separation for 0 Is and N 1s ionization are less dramatic than found for PNA, for tutample

3-Z X&nro&cnzne dimer

In order to theoreticaIIy address the phoroelcc- tron spectra of &id nitrosobenzen~ computations must at least account for the cis4mer pairing as indicated by X-ray diffraction studies [I I ]_ There- fore, spectmI functions of the cis-isomer of the unsubstituted dimer were cakuIated based on a modeI drawn from the experimentaI geometry [Ill: a phuuxr 0 - N-N -+ 0 cis-configuration (&IN0 =X200); 132 A N-N separation; and, phenyl groups rotated 90” reIative to the NO-NO plane. Fig 6 compares the N Is and 0 1s ionization spectra of the cis-nitrosobenzene dimer obtained with and without neutraI ground state correIation_

Fig_ 6 indicates that correlation in the neutral dimer species contributes significantIy to the fiid state dimer shake-up intensities, as opposed to the monomer case where such contributions were found to be - - al_ As indicated in the top panel of fig 6 low-lying shake-up features accompanying N 1s ionization are completely absent when initial state correhition is negkcted. The refative contri- bution of double excitations to the neutral dimer ground state is indicated in table 5

In the case of N fs ionization of the monomer the intease shake-up feature was induced by an interference of phenyl --, NO.. and oxygen --, nitrogen zr-ekctron charge transfer_ Due to the twisted configuration of the cis-dimer charge transfer between f-vents is inhibited in the solid state- A detailed aaxxmt of NO-NO interaction is t.&_css+tial to dcscrii the condensed &ax N Is sh+up stxuctti Analj6is of the 0 1s man* mer spedrum; however, reveakd that phenyI+ NO charge transfer did not participate~as strongly

_.“\ ,N=-N\

CIS - DIMER

~. -. --

.mTL- Wl-IHCNJT~ON i -20 _-lo 0 10

lUITH CDRfEiATIDN

w?AHcwT-ON -20 -lo 0 10 20

- E,IeV)

Fig. 6. Co&par&m of the thcor&aI k 1s and 0 Is corc.hoIe spcmaI functions of the cis.nitrowbGme dimcr obtained with and without ncutraI ground state axrektion. _

in the shakeup process as found for N 1s ionim- tion. Specific NO-NO intera&on should, therefore. be of less consequence to the 0 1s ci+imer speCtraI fun&0n. -This +xhlsion js reflected in fig 6 where corkiderable 0 ls~intensity is also calculated when. ground. state ~rrelation is ne- gh?cted.~. . : .-_:

The wavefunctions of interest fork 1s and 0 1s &+dimer ionization ark given in tables 5 ‘and .7. zspectively. The hxzlization of _relevant dimer orbit& is given in fig_ 7..~Fii the ~intera&ion~

Energies and aa&mctioas of appropriace_N ls’ion state+‘of.,. _*e cis.nitrosobeqzcm dimcr comparcd.to the..wrdatcd PN-i td ground state &ergy relative IO t+ neutral _specjes SF.; gnnkd state Th& ion state IabeIs correspond to +z feaw_ given~che_top~off~6(~capti~ntotablel) -::

gmund state - 1.174 neuvai

ion state 1 -O-61-

ionstate 1.48

O-92 614(o.OIp.O) -03 9s4 (40,4O&al,41)~

0.95661 (o,[email protected]) -0.15 745 (0.37L41.0) ._ - 0.14 405 (37.37jj41.41)

0.20 382 (O.OQct.0) i 0.88 537 (O~TII41.0) +0_14 663 ([email protected]) +033 144 (3737I141.41)

TabIe 6 Overlap intcgnls between ionic detemninants of the N 1s ionized cizxtiumobcmm edimerand(i)thencumldimerSCF deraminanr,<ii)thcwrrrspon~doubl~ucd~ed detmminant

cream3 by (40.401141.41) excitation; the numbering qstem rc. fers to orbitak of La;- net&I dimer SCF determinant

ion wtigumion overlap with OwxIap with doubly. SCF det erminant excited wl&guration of ncutraI ground (40.40~1.41) State

(O.Oip.0) 0.73 394 0.137 171 (0.371f4l.Of - 0.24 150 0.306 534 (0.38&U.O) -O.&t0884 0523 313 (37J7I141.41) 0.079 465 0.6s5009

leading to dimer formation can be understood in terms of fig. 7. For example, the HOMO and LUMO dimer. Ievels are formed by combining the antibonding NO 5 orbitak to yield a bonding and antibonding combination (orbitats *40 and ‘G41~ in fig. 7). Due to steric hindrance these NO levels do r& .&u$e to -the phenyl -ring orbital& ‘specifically orbital .+%21 of fig. 2, as found in the monomer. In the one-electron pi&n-e the dimer is stab& lized by doubly occupying the bonding combina~ tion. Ako, the determinant formed by doubly occupying the antibonding combination is only 4.92 eV higher in-energy. Since the two states have the same symmetry a strong interaction~occurs which Ieads to a ground state .stabiLization of ,1_17: eV_ Subse&ent to &eraction the .grotmd state wave- f&&on achieves ‘approxi.kateXy 11% .doubly &c- dte.d&aracter_ ‘I _- . -_~‘:: . ~-

w3

zlxmnd sc!xc - I_175 O-92 614 (0.9tpm0)

ncuu;tl -0319sJ (40.40~1.41)

ion swtc 1 - 057 O-97 919 (O.qp.0) km scax 7- 4-45 0.53 9s6 (0.36~1.0)

- 022 975 (0.40~~.0)

+ 0.17 939 (0.40@4_0)

io.15 149 (035~Lo) + 031645 (033~1.0)

-0_15 708 (037~.0) -0.16 MO (56Z56pL41)

ioa SUKC 3 67% 022 073 (O_.qpi.O) - 038 624 (039w5.0)

- 031 53s (03S~3.0)

+0_31t40 (033~i.O)

-0-40 315 (0.37p2.0)

+ 0_3S 436 (0_4OpSB;

T&h S

Ion cotlf~umion o\rrhp UiliI O\rrIq with doubk

SCF de-r escitcd configu=ioa

of i?tlIti ground (40.40~1.41)

state

(0_0&0.0) 0.77 763 815 506 (036~1Ln 0201599 - 0245 189

(O.;ropcm - 0.014 902 - 0.001 !ai2

co_4opl.o, - 0.0 573 707 - o_cu7 459

(035Wl.O) - 0_063 742 - om7 459

(033pLo) - 0.16 703 0.1 516 303

(037p4.0) O-006 591 0.0 013 143

(3636W.41) c&o52 ZM 0_3Sl4os

(039gs.o) o_ozO 418 o_OM 1%

Upon creation of an N 1s core hole the niL% gens are no Ionger equivaIent This leads to a rehitive IocaIiration of the one-eIectron wavefunctions on specific ~fragments’* of the dimer_ The occupied orbik& for exampk is Iocahzed on the NO moiety containing the core hole which impiies

a transfer of charge toward the ionized atom. This. is demonstrated more directiy by comparing charge distributions:in the neutral and the N 1s ionic ground state Fig 8 shows gross atomic popula- tions and bond indices for the N2q region A screening charge of O-94 e is drawn toward the ionized nitrogen atom 67% of this charge comes from the non-ionized or “‘neuual” NO ,moiety with the remainder derived from the phenyi group_ Although the occupied and virtual ion levels of interest are Iocahzed on different parts of the N,oZ region. their phase reIatior paraIIeIs that of the neutral system orbit&_ An excited state of the ion. with orbital +41 occupied. is therefore likely to have a small overlap with the uncorrelated neutral ground state The correlated ground state. however. contains a determinant where the antibonding “N,O, orbital” is occupied. The overlap betu-een this component and the excited ion state therefore is expected to be Iarger (see table 6) The resaz!t is that the satellite gains intensity through ground state correlation (see fig 6)

As noted above corresponding interactions within the 0 1s ionized dimer are less pronounced_ The accumulation of screening charge on the ionized oxygen is less (0.49 e) than calculated for N 1s ionization (see fig S). 0 Is ionization, how- ever_ leads to a &n&r locahzation of one-particle wavefunctions as found in the N 1s ionized system The main difference between N 1s and 0 IS ionization appears to -arise from interference terms involving overlap amplitudes between the correlated neutral ground state and components of the ionic ground and excited states_ As indicated in

tables 6 and 8 overlap of the ionic terms with the primary ground state determinant, (O,OIp,O) are in phase for 0 1s ionization, but out of phase for N 1s ionization_ The reverse condition is found for overlaps involving double excitation of the neutral

system Fig 9 compares the ion-state spectral functions

calculated for -the cis-dimw and .a hypotheticaI tmnsdimer configuration~with the e.xperiment.aI

solid state spectra [S]. Computations on-the trans- isomer are induded to provide some indication of the sensitivity of shake-up structure on. increasing the interaction between the NO-NO part and the phenyl groups- Although the unsubstituted trans-

,

Fig_ 7. Ocbiuls of he neulr;+l. N Is ionized and 0 Is ionized uneorrel~~ed ground sta~ci of as r-iewed

from above &e m&euku ptz~ The circles ;irr proportioti IO the LCAO coefficients_ The ionized atom is marked by an asterisk

dimer is not found in the crystal, computations were performed on a construction derived from the geometry of an experimentally confirmed trans-dimer species of a suitably substituted nitrosobenzene [ll]. Our model of the trans-dimer assumed phenyl group rotations of only. 30” from the NO-NO planes The agreement between theory and experiment shown in fig 9 is not as good as obtained for -mtrosobenzene in the gas phase- Cer- tain u trends” on going from the vapor to the solid are, however. adequately mpresentecIby the cis-dimer model For example, the relative 0 1s shake-up intensity isdecmased from 62% to 48% upon condensation [7,8) in .comparison to the ScaIculated

-decr& of 52% to 26%. In addition, the computations indicate a 13 eV increase in satellite energ_V on going to the solid compared to the experimen- ml value of 0.25 eV. Although computationsindi-

c6% C&5 -.

Q :o

: 1 Ftmybbitais

Q-37

M

0

Q36

i7_i95 eV

/ C&s C;1&

0 35 Phenytorbitat

9

Q3c

18.262 eV

cate a 0.5 eV shift-in the N 1s shake-up peak to lower energy with an accompanying loss of intensity on going to the solid which is contrary to experiment, the relative influence of dimer formation on the N Is and 0 1s shake-up energies is correctly given.

Corresponding studies on PNA [2.3,6] and 2- amino&nitronaphthaene [6] have. shown quite good agreement between caIcuIated and experi- -mental differences in heteroatomic core-hole structure on going from the vapor to the condensed phase_ It is worthwhile to comment on. possible origins of .the generally less impressive cornpa& sons obtained in the present anaiysis. For example, it is possible that when the solid is prepared

‘by condensation of the nitrosobenzne vapors onto ~ a _cooled substrate the dimer is incompleteIy formed, or that-a precursor smte is achieved with

‘226 K-L FieuudmaL /Shak&ups lrxcnue of core-ioniierisuhvinmd anmuuic mdeda

CIS-DIMER

NEufFuu_ 65735 0

B!33!5 0

Nls

07s

an enlarged N-N separation_ A careful X-ray photoelectron spectroscopic study of w&I characterized nitrosobenzene mokcuIar crystals would provide conchrsive information on dimeric contributions to illttXIIl0kCUkS screenhg. Consideration of a singIe dimer pairing may also be inadequate A theoretical aspect likely to he of even greater significance is the lack of size consistency in the CI evaluation of tfic monomer versus the dimcr structure_ The number of excited configurations included in the monomer and dimer computations were identicaI,

ie 300 singly and doubly excited configurations_ However, since the number of orbitaIs are doubled on going to the dimer. certain higher-lying excitations addressed in the monomer computations are neglected in the Iarger system. The excited dimer states are therefore not as-completely descrii as those in the monomer Inclusion of a larger number of configurations in the dimer problem may well influence the reIative shake-up intensities and energies_ The problem of size consistency is further addressed in the following section_

3.3. NO-tiitner

In order to reduce the magnitude of the size- wusisteucy problem and to address the issue of “partial” dimer formation upon condensation of nitrosobenzene, corresponding computations were performed on the nitric oxide dimer, (NO),_ Ne- glect of the phenyl groups appears to be a suitable approximation in a detailed analysis of the “local- ized” NO-NO interactions_ Although some screerkg charge was drawn from the aromatic moieties in the 0 Is and N 1s ionic ground states of the cis-nitrosobenzene dimer, exciraz~onr involving the phenyl group orbitals were found to have a negIigiiIe effect on the resulting shake-up struc- tures_ The comparison computations noted above on the trans-nitrosoben&e dimer further indicates an absence of significant phenyl group contributions to the satellite structure, i.e. a ro- tation of the phcnyI groups toward grcatcr planar- ity with the NO-NO projection yiekls an 0 Is shake-up structure without a distinct peak- This rest& is contrary to experiment as shown in fig. 9. Although the one-electron coupling within the NO dimer is found to be somewhat different from the NO-NO coupling within the.nitrosobenzne dimer, sufficient paraIMs exist to permit us to qualitativeIy examine spectraI consequences of part&I bond formation in the latter system by vatying the bond distant in the NO dimer.

Fig_ 10 showsthe caIcuIated N 1s and 0 1s ho&state spectra of (NO),- for the indicated equi- librium geometry [15,16J9-31]_ The i.niIuence of doubly- excited states is CicarIy demonstrated: As indicated in ~fig_ 11 a rather WZ$C m. bond is formed by interaction of the in-pIaue components

_ J,_ T-S-DIMER . ~~~

-20 -10 0 10 20 -20 -10 0 i0 20 J%L

-eV -Es

eV

of the two singly occupied 2r;* orbitals of orbitals of fig 11). The reverse charge-transfer in NO j13,1,4]_ The orbital corresponding tc, the (J the N 1s shake-up state amounts to 56% of the bond constitutes the HOMO Level whereas the u* ground state screen&g charge_ This result is in virtual leveL derived from bonding interactions essential agreement with the conclusions of TO&S becomes the dimer LUMO level_ The two out-of- et aI_ [13]. Tables 9 and 10 reveal that the large plane 2r;* components form s orbitak and are not influence of initial-state correlation arises from the occupied_ Upon ionization a screened hole state is Iarge overIaps invoking the doubly-excited config- formed as exemplified for the N 1s core hole in fig uration contributing to the neutral molecule ground 12a_ l-he total screening charge of 1.08 e accu- state where the u-character antibonding 2%* orbital mulated on the ionized atom is derived from the is occupied. adjacent oiygen atom (24%j and the “neutral’ At this point the correspondence between the NO moiety (7695 This characterizes the ionic NO and nitrosobenzene dimers becomes obvious: ground state of the NO dimer as an intermolecular if the geometry of the NO dimer is changed simply screened state, similar to the situation found for by eniarging the intermolecular separation the en- the nitrosobenzene dimer__The charge distribution ergy and i+nsity of the primary N 1s shake-up corresponding to the rather intense N is shake-up feature is modikd (as indicated in fig 13). The featureobt&ned when neutral ground state correlation is included is shown in fig 12b.- clearly,

right-hand panel of fig_ 13 grapbieaUy shows the. variations _for the two core-hole lines ~resulting

charge- is transferred bath ‘to the “non~ionized’ moiety through the 2=* channel (see the ion-state

from N 1s ionization_ As the interaction between NO moieties becomes less, *he splitting between

snurpd=w -1560 o-92 875 (O.Ogw> ncU& -e.33 336 (1131gl2.12)

ionsxacl -122 -0-97 31s (0.0go.o,

10.x3 299(1OJO~434)

ion stare 2 3z2 O-94 133 (O_ll[p39) +025 362_(llJl~I3J3)

ionsme 1x29 -019 182 ([email protected])

i- 020 309 @qp2.0) -o_l233O(lOJlp3.l4)

-0.62 8(p (O.lO1p4.0)

-0-40 459 @.6@3a, -0-37 164 (0.5@3D)

-020 5% (lO.llfp3.l4) + o_ls 215 (0_4p3$3)

T&k 10 Ow&ap integais betam ionic det elxelants of the N 1s

ion&d &-nitric oxide dimer md (i) rhe neutral dimer SCF

dcmrminant. (iii the cot-r+- doubly excited detaminanr

-ted by (ll.ll~2.12) excitation; rhe numbering sy~rem m-

furs ~1 orbiuls of he ncuval dimer SCF determinant

ionconfiiumtioo Oe&pwith Ovaiapaithdoubk S&da cllx&unt acited configuration

neutral ground of (I131~212) slate

(o.OiP.0~ 0.7lo 84s 020302

(10.10p4.14)

(O.llul3.0) (ll_llj113.l3)

qL9yr3.0,

COstpro, (iO.llftl3.I4> ‘(0.10~14.0)

(0.6fl3.0) : Gml3.0) (0.4913.0)

(Iomp314)

0.023 687 o.an 341 - 0.357 381 03WF8 _

0.179675 0_7Q7 436

-0ms 326 ems 901

-0_012222 ~0_003788- 0_065238 : -0_072%4 .~

-0J.29 761 -0.010215 .. 0.019 965 -0.0193 629

-0_06i 931.

: -0_0720641

0.0 004 224 o-0 a1308 --- 0.065238 -0_070641 ~:

-ELECTfiON-DEN+

. .

6.X03 6X03

.J /

2.8597 _ a597

-1961 eV

-U).LOZ eV

ORBIT4LS -. : . .

-lo.806 eV I lnncr

_ -20.6&L eV

-20.965 eV

Fig. 11. Orbital locjlizations for the uncorr&tcd neutral and N Is ionized ground states of tk nirric o-side dimcr Y viewed from abow the NZO, plant Atomic ckmon densities as well as bond orders for the two states are aIso &x_

features is reduced concomitant with an increase other, i-e_ as in a-nitrosotoluene where CH2 groups in satellite intensity. At larger separations the “isolate” the NO groups from the phenyl rings- satellite corresponds to an inrramokcularly the total binding enthaIpy increases dramaticaIly: screened state. The satellite should thus gain in- 0.05 eV versus 0.87 eV [9]_ On the basisof this. tensity as the NO moieties achieve greater ‘Lmono- result a rather smalI nitrosob enzene dimer satellite mer-Iike” character_ When the intemction is rather intensity is expected which is neither observed or strong the satellite intensity is reduced since in- calculated (seefig 9).‘This apparent ambiguity can temrolerular scmeningdominates

Since the NO dir& and nitrosobenzene dimer be resolved through a ~comparison~of the relevant dimer orbitak As noted’ above,. fig- 11 clearly

satellite intensities are experimentahy similar, a shows that the (NO), shake-up occurs within the . . simiIar ON-NO bond strength is expected in the;:. manifoId of u orbitals formed by .veak inter&on~ two systems. A compkison of heats of dinkiza-. ’ of the two in-plane 2k orbitak The “weakne&’ of tion indicates that this is indeed -the case: O-05 eV .- for the nitrosohenzene dimer [9j, and 4~07 eV. for

this coupling determines the satellite intensity_ On the other hand, the . cr interactions between

(NO)1 [l&16]_ In the- case of the ~mtrosobenzene mtrosobekene moieties is much stronger due to dimer, however, the total binding energy is a suf .I -the ihorter N-N bondlength The main contribu- per-position of the NO-NO -interaction, and- the- 1 tions to the nitrosobentene~ dimer shake-up, how- d&abilizing interaction involving the pheny1 rings_ ever, lare ~5~ electrcks as outlined above The e&t- If the phenyi groups are assumed to avoid each -- phng between the 2; ,?rbita& in. the: dimer is .-

Fii 12 E.latma dalsiry cliff- plotsoftkgound(a)and nusst intense shake-up (b) state of N Ls ionkcd (NO),. The maps are pbtvd in ck NO-NO plaza

. indeed much weaker than the aazompanying~ Q’

interaction_ The NO-NO G orbital coupling in the ~~ nitrosobenzene dimer is, -in fact, comparable in

magnitude to the u couphng in (NO),. The inters actions leading to the high core-hole satehite intensity observed for the nitrosobenkene dimer thus parallels the analysis of the (NO), shake-up features, in spite of the shorter bondlength achieved in dimerfcumation- Variation of the magnitude of ON-NO coupling within the nitrosobenxene dimer would prod&e results similar to those shown in fig 13 for (NO),.

Further evidence indicating the significance of “interrnoIccuIa? ekctronic coupling between components of the NO dimer is given by the appearance of new features in the valence ionixa- tion spectrum on going from the vapor to the condensed phase 113,321 (see fig 14) Con- densation yields a reduced itttextsity peak = 22 eV above the first ionization band which is absent in the :s phase_ In accord with experiment open shell CNDO/S CI computations on the valence level ion states of (NO), yields a weak intensity, or shake-up feature, at = 32 eV above the first ionization (see tig 14) In line with symmetry arguments [33]. the computations indicate that the solid state induced Iow-Iying shake-up arises from coupling between the HOMO ---, LUMO excited

-L1-)u-- rg\c=, - -.

K&N IAl-

Fig13_Thealagics a&intcnsi~ofthctaomostincmsefeahlrcs of Lhe (NO), N Is core-hole spearal funcrion as a functiori of

intermoknJar&= :

NO solid (on Au1

TONNER ET AL_

(NO), THEORY

. .

: -..

: 1.

wnfigumtion and the second primary hole St&e obtained by ionizing the second highest occupied dimer orbital The HOMO and LWMO orbitais of the valence-ionited dimer are generated from wu- pling behveen the in-plane NO 2=* components. and thus correspond to those Ievels which are of particular importance in the elucidation of wre ionization- The primary hole configuration from which the vaience level shake-up state of interest receives intensity is likewise derivd from ionization of an in-piane orbital Further analysis of the valence speerrum is in progress [34]_

4 Summary and condusions

Theoretical N Is and 0 1s core-ionization spee- tm of monomeric nitrosobenzene and a model nitrosobenzene dimer structure =xre cakulated within the CNDO/S CI equivalent-core approximation with ahowance for up to doubly excited cotigurations and favorabIy compared to existing vapor and condensed phase spectra_ Unlike the dramatic differences in core-hole structure observed for some donor/acceptor-substituted aromatic systems on going from the vapor to the wndensed phase. the satellite structures for N Is and 0 Is ionization of nitrosobenzene are r&a- rice& unchanged upon condensation even though specific i~2fernrufecu&r interactions are experimentaliy indicated for the solid_ Our analysis indicated that such a similarity was uaccidental” due to different wuphng mechanims active in the two eases

Specifically. the theoretical results for the planar monomer ionizations were in excellent agreement with observation_ Intense satellite structure for both 0 Is and N 1s ionization, at the approxi- mately correct energi~ were obtained_ The nature of the sateIke feamres were anaiyzed in terms of interactions between the arotnttic and nitroso groups- Neutral molecule ground state correlation had little effect on the fmal state spectra_ Compu- tations on the dimer structure provided less quantitative agreement with the condensed phase spectra. although subtle energy and intensity variations noted experimentalIy on going to the wndensed phase were quahtatively reproduced_ The

. dimer geometry used to model the condensed phase environment was derived from the kperimental vscai srrucncre where specific NO-NO his-dimer pairing was indicated. with the phenyl groups rotated = 90” out of the NO-NO plane_ The less than ideal agreement between theory and experiment pointed out the importance of a careful X-ray photoelectron spectroswpic study on molecular czystds of the nitrosober&ne dimer (di- azobenzene dioxide) to unequivocally demonstrate the infiuenee of well characterized EntermoIeeular interactions on the core-hole structures. Computa- tions. nevertheless, indicated that solid state shake- up arises from inrefnzolecuhr screening betu-een the coupled NO moieties, and is thus quite different in origin than found for the gas phase species. The importance of neutral ground state correlation in the description of the condensed phase spectra was also demonstrated. Comparison wmputations on a hypothetical trans-nitrosobenzene dimer. where the phenyl moieties were only rotated 30° from the NO-NO plane, further verified the lack of significant aromatic group contributions to the condensed phase shakeup structure.

To further elucidate the wnsequences of “IoeaI- ized- NO-NO coupling on the nitrosobenzene condensed phase spectra the N 1s hole-state sperm- b-al function of the nitrous oxide dimer was ex- amined- Intense satellite structure was attributed to intermolecular screening factors in accord with prior theoretical arguments_ Ahhough relative satellite intensity was shown to be strongly depen- dent on N-N separation, the fina state intensity and energy distribution cahxrlated at the equi- librium geometry was in close agreement with experiment_ Parallels between the nature of the coupling in (NO), and in the nitrosobenzene condensed phase and crystal habit were indicated.

Further evidence indicating the significance of “intermolectdar eiectronic coupling between components of the NO dimer was obtained through a comparison of the existing UPS condensed phase spectrum with semi-empirical open shell computations in the valence !ev& ion states_ Orbitals derived from solid state interaction, and which wn- tribute to the experimentally well-resolved valence level shake-up peak, were found to wrrespond to those Ievels which w-ere of particular i&port&x in

Mcssmer and W-R Sala&& Ph>;s. Rty. ~~ttcrs 51(1983).- 137s

the determiiatik of the-core-hoIe shake-up features.

Finally, the present study emphasized that analysis of solid state coke-hoIe speka baked on corn-. putations restricted to an isolated molecuIe model

- may Iead to misleading interpretations.

xdulovrkcIgeinent

We thank the Regionaks Rechenzentnun der L’niversit% zu K6ln for computer time and the Fonds der chemischen Industrie for financial sup-

port-

References

[l] MS Banna. Chcm Ph>s_ 45 (1980) 383. [l] RW. Bigelow and Ii_-J. F-d. Chctn. Ph_\-si Letters 77

(1961) 26L [3] H.-J_ F-d and RW_ Bigclow. Cbem Phys 55 (1981)

407_ [4J H. iiSmn. B-D_ Roes. PS. Bsgus. U. Gelius. P. Xfdmquist

S. Sremon, R Maripuu and K SieS.bahn. J. Chem. Ph>s 77 (1982) 3893.

[S] S Pi~nataro and G_ Distefano_ J. Electron Spcctty. 2 (1973) 171.

[6) H.-J_ F-d. AR !5lauSbter. S&L BalIina MS Bans. RW_ BiSelow. B. Dick. J. Le.. and K&f_ Degx. J. Chem Ph>x 8X(1984) 2535.

[7] AR Slaughter. MS. Bamm and CA McIhwzll. Chem. Ph>s Letters 98 (1983) 53L

[S] G_ Distefano. D. Jones. A MocIelIi sod S. l’ig=txo. Physica Scripta 16 (1979) 372; C&n Ph>. 52 (1980) 3M.

191 B.G. GowznIock and W_ Litttke. Quart_ Rev_ 12 (1958) 321_

[IO) W_ L0ttke. Z El&trocbem_ 61 (1957) 302.976. [Ill DA Dieterich. 1-C PauI and D-Y. Curtin, J. Am Chem

Sa % (1974) 6372

1141 CJ. Nelin. PS Bzg& J_ Bdun and CR Bnmdk. +m_. Ph>x Letters 105 (19S4).5S_

[15) J. Billingslcy and AB. Callear~~rans F-day: e 67 (:971) 5s9_

1161 W-v_ Lipscomb. EE Wang, WX May ,d g lippert. Acra Cest 14 0961) 1100.

[17j J. DeI Fkne and I-LI-L Jaff& J_ Chem Phyk 48 (l96S)_&. 4050: 49 (1968) 1221.

IiS] R_L Ellis G_ Kudmicnz and l-MI_ Jaff&, Theoret C&m_ Acta 26 (1972) 131.

1191 B. Dick and G. Hoblncicber. Tbeoret. Chim_ Acta 53 (1979) 221.

[ZO] R Pariser and RG. Parr. J. Cbcm Ph>s. 11 (1953) 76h [Xl W-L_ Jolly and D-N_ Hendrickson J. Attt. Chun Sot 92

(1979) 1863. [22] D-W_ Davis and DA Shirley. Chem. Php. Lettas 15

(1972) 185.

j23] I-L-J. Freund. H. PuIm. B. Dick and R Lange. Ghan Ph>= 81 (1983) 99.

[24] KM Chang. H.H Jaff& and CA Masmtidis J. Phjx

Cbem. 79 (1975) 1118. [Xi] RW_ Bigeloa; Chum Phys 80 (19S3) 45.

[Z6] JA Pople and D-L. Bcveridgc. Appro.ximate mokcular orbital tbcory (_McGraxv-HI-. New York. 1970).

(2771 I. Cohen. J. Chem. Phys 57 (1972) 5076. [28] B. Dick and I-L-J. F-d. Intern_ J. Quantum Chem_ 24

(1983) 747. 1291 W_i_ DuImqc E_A Meyer and W-X Lipscomb_ Acta

Crjw_ 6 (1953) 76s

[30] CJ. Kg. P-W_ Fricdemann. B.H Mahan ad Y-T_ Lee. J_

Chem. Phyx 66 (1977) 3985.

[31] J. Suzann e. P. Coulomb. M Bier&it, M. Mate&i. A Thorny. B. CroseIt and C_ M2uti. Ph>s. Rev_ Letters 41 (1978) 760.

[32] O_ Edquist L &brink and E J._indholm. Z Naturforsch- 269 (1971) 1407.

[331 G_ HohIneichecr. I? DistIer. M Mi2Ier and H-J_ F-d.

Chem. Phys. Letter, ; 1(19&i: 15L [34] RW_ BigeIoa- and 1 --J. F-d. in preparation

19s) 215-233 :. -- : - -.~ , . . .- -,;: .-.. .- . . :- .-, .,: -, ‘25; :- 1...

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